DFT Study of Closed and Open Sites of BEA, FAU, MFI, and BEC Zeolites Substituted with Tin and Titanium

DFT with long-range corrections and ONIOM along with a polarizable-continuum model were used to analyze zeolites BEA, FAU, MFI, and BEC substituted with Sn and Ti. The preferential substitution sites for Ti and Sn in the different frameworks are reported. The Lewis acidities were measured through the NH₃ binding energies and through the charge transfer of NH₃ upon adsorption. The deprotonation energies of the open sites, which are proportional to the Brønsted acidities, and the hydrolysis energies are also reported. We also present the properties of BEA with a single and a double Sn-substitution to compare the active sites obtained with two methods commonly employed for the synthesis of Sn-BEA. Among the zeolites analyzed in this study, Sn-BEA with a double Sn-substitution has the highest Lewis acidity. The formation of open sites through the hydrolysis of Sn-BEA, Sn-FAU, and Ti-FAU is energetically favorable, but it is not favorable in MFI or Ti-BEA. On the basis of the deprotonation energies, the open sites of Sn-BEA have a strong Brønsted acidity, comparable to Al-BEA or Al-MFI. We also demonstrate that the VDW forces in the binding energies of NH₃ on MFI are more significant than in the other zeolite frameworks and that these forces decrease with increasing pore size.